High gloss coated papers known as cast-coated papers are prepared by applying an aqueous coating color based on a pigment and an adhesive on the surface of a base paper to form a cast coating layer, and pressing and drying the cast coating layer in the wet state against a heated mirror finishing metal surface (drum).
Known processes for preparing such cast-coated papers include the wet casting method involving directly pressing a coating layer in the wet state against a heated mirror finishing surface to give a gloss finish; the gel casting method involving gelling a coating layer in the wet state and pressing the gelled layer against a heated mirror drum surface to give a gloss finish; the rewet casting method involving drying a coating layer in the wet state, and then plasticizing the dried layer by rewetting and pressing it against a heated mirror finishing surface, etc.
All these processes for preparing cast-coated papers equally involve pressing and drying a cast coating layer in the wet or plasticized state against a heated mirror finishing surface. However, they have the following disadvantages relating to coating runnability and quality of the resulting cast-coated paper depending on the plastic state of the coating layer. In the wet casting method, the temperature of the mirror drum surface cannot be 100° C. or more because the cast coating layer has low viscosity causing the coating color to boil and the coating layer being broken when the temperature of the mirror drum surface reaches 100° C. or more. The absence of a drying step before casting increases the drying load, resulting in low speed operation.
In the gel casting method, the temperature of the mirror finishing surface can be 100° C. or more because the cast coating layer is gelled. However, the absence of a drying step before casting also increases the drying load and requires that a lot of water contained in the cast coating layer should be smoothly transferred into the base paper layer and evaporated off when it is contacted with the mirror surface drum and moreover, sheet gloss or other quality decreases during casting at very high speed because the gelling degree of the coating layer is difficult to control.
In the rewet casting method, the temperature of the mirror drum surface can be raised to 90-180° C. because the cast coating layer is dried before casting. However, this method has the disadvantage that defects on the so-called cast-coated surface such as pinholes on the cast coating layer surface or uneven adhesion are liable to occur during high speed casting because the plasticity of the cast coating layer is lower than obtained in the wet casting or gel casting method.
In the aspect of qualities of cast-coated paper, print gloss is normally lower than sheet gloss, and therefore, print gloss as expected from sheet gloss cannot be obtained in full-page prints and further improvements in print gloss and cast-coated surface quality would be desirable.
In order to solve these problems, various methods have been proposed. For example, it was proposed that a plastic pigment and a latex having a minimum film-forming temperature of less than 0° C. be added to the cast coating layer (see patent document 1). The cast-coated paper obtained by this method has good sheet gloss, but suffers from low print gloss, insufficient air permeability of the paper and low production efficiency. Another proposal was to define the particle size distribution of the pigment in the cast coating layer (see patent document 2). In the cast-coated paper obtained by this method, print gloss is improved over prior products but is low relative to sheet gloss, and the cast-coated surface quality is also poor. Still another proposal was that a hollow plastic pigment be added to the cast undercoat layer (see patent document 3). The cast-coated paper obtained by this method has improved production efficiency over prior products, but qualities such as cast-coated surface quality and printability are not sufficiently satisfactory.    Patent document 1: JPA HEI 4-146294.    Patent document 2: JPA HEI 10-18197.    Patent document 3: JPA HEI 9-268493.